Question

I am trying to implement a tailable cursor for mongo using C driver. Uptil now i have been able to create it and successfully get the pushed data into my process with the following code

cursor =mongo_find( connection, DB_TENANT_NAMESPACE, query, bson_empty( &e ), 0, 0, MONGO_TAILABLE | MONGO_AWAIT_DATA);
while(1)
{
    while(mongo_cursor_next(cursor) == MONGO_OK)
    {
       b=mongo_cursor_bson(cursor);
      if(bson_find(iterator,b,"_id"))
      {
        oid =bson_iterator_oid(iterator);
        bson_oid_to_string(oid,&id);
        printf("ID:%s\n",id);
      }

}

With this code i can get the updates. But looking at the tailable cursors docs, it seems that i need to run the mongo_find inside the outer while loop to make sure i get the latest entries. The docs suggest appending to query with gte. Copying from docs

query = QUERY( "_id" << GT << lastId ).sort("$natural");

The issue is that the oid is an object which can be converted to a string. I dont really think i should be converting it to an int in-order for gte to work. Any ideas?

Answer 1

ObjectId’s may be logically compared by those operators, as can Date and Timestamp objects. There should be no need to represent the ObjectId as a string, and is no practical reason (at least in this case) for comparing an ObjectId to a string.

Note that comparisons involving two different BSON types will follow this compare order.